This invention concerns a protection circuit for a superconducting magnet. In particular, it relates to a protection circuit for protecting the superconducting magnet from damage which could otherwise occur during a quench. A quench occurs when a superconductor, such as used in superconducting magnets, reverts to a resistive state. This may be caused by localized heating in one part of the superconductor. A small part of the superconductor ceases to be superconductive, and enters a resistive state. Any current flowing through the resistive part will cause local Joule heating. This will cause the adjacent parts of the superconductor to quench, resulting in a larger resistive volume, in turn causing further Joule heating. Very rapidly, the superconductor enters a resistive state, with a potentially very large current still flowing.
Prior to the quench, coils hold a large stored energy, which may be in the order of mega joules. Following quench, this will be dissipated in the resistive volume of the conductor. If the quench process is not adequately managed, this energy can be dissipated in confined areas, resulting in local temperature rises which can damage the coil areas at or near the part where the quench was initiated.
It is known that harmful concentration of heat may be avoided by spreading the quench process, so that the resulting heat is dissipated over as much of the available superconductor as possible. This will result in a quench involving substantially the whole of the superconductor, meaning that no part should reach a dangerous temperature. In a superconducting magnet, such as those used in MRI or NMR imaging systems, this is typically achieved by deliberately initiating a quench on the superconductor coils, other than then the coil where the quench started. The deliberate quench initiation is typically achieved by applying a current to heaters, which are in close thermal contact to the coils. Typically, each coil will be fitted with two or more heaters.
FIG. 1 shows a circuit diagram of a known quench protection circuit, suitable for installation in superconducting magnets of MRI or NMR imaging systems. A superconducting magnet 10 is represented, comprising coils L1-L6 connected in series. Each of these coils has a corresponding heater R1-R6 in intimate thermal contact. This may be achieved by gluing the heaters on the surface of the coils. The heaters are electrically connected in series, and this series arrangement is connected in parallel with a subset L2-L5 of the superconductor coils. Current injection leads 12 are provided, one to each end 14, 16 of the series connection of superconducting coils. The ends 14, 16 of the series of superconducting coils are connected to a cryogenic switch 18.
All superconducting magnets which are operated in the so-called persistent mode have a cryogenic switch. Essentially, it is a piece of superconductor wire, in series with the magnet coils, with a heater attached to it. If the heater is on, the cryogenic switch 18 is normally conducting and is open. When the system is attached to an external power supply by leads 12, current will flow through the superconducting coils 10, with only a trickle running through the cryogenic switch 18. When the magnet system is ‘ramped’ to the required current, the heater is turned off, and the cryogenic switch 18 becomes superconducting: the cryogenic switch is closed. As the external power supply connected to leads 12 is ramped down, the current through the cryogenic switch 18 will increase by the same amount as the decrease in the current through the external power supply. Once the external power supply is ramped down completely, the current leads 12 may be removed, to limit heat leakage into the cryogenic magnet system.
The ends 14, 16 of the series of superconducting coils are connected by a diode pack 20. A similar diode pack 22 is in series with the heater R1-R6. In each of these diode packs 20, 22, two series connections of two diodes are placed in inverse parallel.
Diode pack 20 protects the cryogenic switch 18. To illustrate the protection offered by the diode pack 20, consider the situation when the magnet current is being ramped. The cryogenic switch 18 is open and a current of, for example, 500 A is flowing through 30 the coils L1-L6, the leads 12 and an associated power supply unit. If for some reason the current is interrupted, in the absence of diode pack 20, the inductance of the coils will act to force the 500 A current thorough the cryogenic switch 18. During ramp-up, this switch would be which in its open state, with a resistance in the region of 30Ω. This would generate a heat dissipation of up to 500 A*500 A*30Ω=7.5 MW, sufficient to destroy the cryogenic switch 18. In the presence of the diode pack 20, the diode pack 20 will become conductive as soon as the voltage drop across the cryogenic switch 18 exceeds a threshold voltage of the diode pack. This will occur at a relatively low voltage across the switch, before the current in the cryogenic switch 18 has risen sufficiently to cause any damage. In order to maintain the diode pack in a non-conductive state during current ramp-up of the magnet, the threshold voltage of pack 20 should be slightly higher than the ramping voltage L.dI/dt, where L is the inductance of the, magnet coils, and dI/dt is the rate of increase of the current through the magnet coils. For example, the voltage across the coils may be ramped at 10 Volts, with the current increasing accordingly.
Diode pack 22 stops the heaters R1-R6 from conducting during current ramping of the magnet coils, as its threshold voltage is selected to be greater than ramping voltage L.dI/dt. The diode pack will become conductive, allowing the heaters to function if a higher voltage develops across the magnet coils, for example during a quench.
When one of the coils L1-L6 quenches, a voltage will appear across that coil, and so also across the series connection of resistors R1-R6 and diode pack 22. This voltage rises rapidly in time, as the quench propagates within the coil. When a certain threshold voltage has been achieved, diode pack 22 will begin to conduct. A current ir starts to flow through the heaters R1-R6. These heaters will then initiate quenches locally in each coil L1-L6. By initiating quenches in all of the coils, the energy to be dissipated in the quench is spread relatively evenly across all of the coils, with the intention of avoiding any one coil heating enough to be damaged.
This arrangement has at least two drawbacks. Firstly, the voltage generated by the quench can achieve high values, causing high currents and dissipation in the heaters, which can result in the destruction of the heaters. The alternative to this would be to provide more massive heaters, or higher resistance heaters, which in turn would introduce a time delay in heating. Heaters of increased resistance would supply heat at a reduced rate, but in doing so would slow the spread of the quench, and so may increase the risk of damage to the coils. More massive heaters have a larger thermal inertia, and take a relatively long time to heat up. Ideally, the heaters are required to heat instantaneously as soon as a quench has occurred anywhere in the magnet. Secondly, the threshold voltage for operation of the heaters is determined by the forward voltage of the diode pack 22, which can be higher than desirable. This may not begin to conduct soon enough, and the risk of damage to the coils may not be fully eliminated.
The present trend is for superconducting coils to become smaller, more compact, and carrying higher currents. This increases the need for rapid quench propagation, since an increasing amount of stored energy needs to be dissipated in a decreasing volume of superconductor.
U.S. Pat. No. 5,278,380 describes inductive quench heaters for a superconducting magnet system.